Adhesive labels of the type under discussion have basically been known for a long time (see German Laid-open Applications DE-OS No. 34 17 746, 25 46 108 and 15 94 060). Adhesive labels which can be removed again, i.e., which can be removed after having been glued on once, make special demands on the chemical composition of the several layers of such an adhesive label, especially on the chemical composition of the adhesion layer. The printing carrier, which can consist of many types of material, though mostly of paper, and which represents the real surface of the adhesive label, in most cases can only be mechanically stressed to a limited degree and therefore requires the adhesion layer to have a low peeling value. At the same time, the adhesion layer must optimally stick on the substrate on which it is glued. The material of the adhesion layer must, on the one hand, be very free-flowing in order to peel off an adhesive label with as little residue as possible, and, on the other hand, the material must also show good internal cohesion. The peeling value also should not be permitted to increase too much in respect to the duration of adhesion, but should optimally be as constant as possible. Finally, the adhesion of the adhesion layer to the printing carrier must be greater than to the substrate on which it is glued, regardless of good adhesion to the substrate on which it is glued.
It is precisely the special demands for a removable adhesive label that have led to a special interest being given to the primer attached to the back of the printing carrier. In the final analysis, the previously noted requirements make considerable demands on this primer. The state of the art works with a primer applied in the liquid phase which, after brief initial drying, is covered with the adhesive of the adhesive layer, also sill in the liquid phase. Primer as well as adhesive are materials dissolved in highly volatile liquid hydrocarbon solvents.
A special problem of the known adhesive labels is their tendency to age because of migration of molecules from the adhesive layer into the printing carrier and because of chemical changes in the adhesive layer. A further considerable problem is the use of environmentally burdensome solvents for the primer and adhesives of the adhesive layer. Finally, the cost effectiveness of the known removable adhesive labels is not yet satisfactory.